1. Field of the Invention
This invention relates to organic polymer light-emitting diodes with improved luminous efficiency and improved radiance.
2. Description of the Related Art
Diodes and particularly light emitting diodes (LEDs) fabricated with conjugated organic polymer layers have attracted attention due to their potential for use in display technology. A standard polymer LED architecture includes the following layers in contact sequence: a substrate with a coating of indium-tin oxide (ITO), a passivation layer, emissive polymer, followed by a single-layer cathode. [See for example, Burroughs, J. H., Bradley, D. D. C., Brown, A. R., Marks, R. N., Mackay, K., Friend, R. H., Bums, P. L., and Holmes, A. B., “Light-emitting diodes based on conjugated polymers,” Nature, 1990, 347, 539; Braun, D., and Heeger, A. J., “Visible light emission from semiconducting polymer diodes,” Appl. Phys. Lett., 1991, 58, 1982.]
In the field of organic polymer-based LEDs, it is common to employ a relatively high work function metal as the anode, which serves to inject holes into the otherwise filled π-band of the semiconducting, electroluminescent polymer. Relatively low work function metals are preferred as the cathode material, which serves to inject electrons into the otherwise empty π*-band of the semiconducting, electroluminescent polymer. The holes which are injected at the anode and the electrons which are injected at the cathode recombine radiatively within the active layer and light is emitted. Proposed criteria for suitable electrodes are described in detail by Parker, I. D., “Carrier tunneling and device characteristics in polymer light-emitting diodes,” J. Appl. Phys., 1994, 75, 1656.
Typical relatively high work function materials for use as anode materials include transparent conducting thin films of indium/tin-oxide (see, for example Burroughs, J. H., Bradley, D. D. C., Brown, A. R., Marks, R. N., Mackay, K., Friend, R. H., Burns, P. L., and Holmes, A. B., “Light-emitting diodes based on conjugated polymers,” Nature, 1990, 347, 539; Braun, D., and Heeger, A. J., “Visible light emission from semiconducting Polymer diodes,” Appl. Phys. Lett., 1991, 58, 1982.) Alternatively, thin films of polyaniline in the conducting emeraldine salt form can be used (see, for example Cao, Y., P. Smith and Heeger, A. J., U.S. Pat. No. 5,626,795; G. Gustafsson, G., Cao, Y., Treacy, G. M., Klavetter, F., Colaneri, N., and Heeger, A. J., “Flexible Light-emitting diodes made from soluble conducting polymers,” Nature, 1992, 357, 477; Yang Y. and Heeger, A. J., “Polyaniline as a transparent electrode for polymer light-emitting diodes: lower operating voltage and higher efficiency,” Appl. Phys. Lett., 1994, 64, 1245; Yang et al., “Bilayer composite electrodes for diodes,” U.S. Pat. No. 5,723,873; Yang, Y., Westerweele, E., Zhang, C., Smith, P., and Heeger, A. J., “Enhanced performance of polymer light-emitting diodes using high-surface area polyaniline network electrode,” J. Appl. Phys., 1995, 77, 694). Thin films of indium/tin-oxide and thin films of polyaniline in the conducting emeraldine salt form are traditionally preferred because, as transparent electrodes, both permit the emitted light from the LED to radiate from the device in useful levels.
Typical relatively low work function metals that are suitable for use as cathode materials are metals such as calcium, magnesium, and barium. Alkali metals tend to be too mobile and act to dope the emissive layer (e.g., electroluminescent polymer), thereby causing shorts and unacceptably short device lifetimes. Alloys of these low work function metals, for example, alloys of magnesium in silver and alloys of lithium in aluminum, are also known (see, for example, VanSlyke, U.S. Pat. No. 5,047,687; VanSlyke, S. A., Tang, C. W., U.S. Pat. No. 5,059,862; Heeger et al., U.S. Pat. No. 5,408,109.) The thickness of the electron injection cathode layer typically ranges from about 200 to about 5000 Å (see, for example, VanSlyke, U.S. Pat. No. 5,151,629; Friend et al., U.S. Pat. No. 5,247,190; Nakano et al., U.S. Pat. No. 5,317,169; Kido, J., Shionoya, H., and Nagai, K., “Single-layer white light-emitting organic electroluminescent devices based on dye-dispersed poly(N-vinylcarbazole),” Appl. Phys. Lett., 1995, 67, 2281.) A lower limit of about 200 to about 500 Å is required in order to form a continuous film (full coverage) for a cathode layer (see, for example, Holmes et al., U.S. Pat. No. 5,512,654; Utsugi, U.S. Pat. No. 5,747,930; Scott, J. C., Kaufman, J. H., Brock, P. J., DiPietro, R., Salem, J., and Goitia, J. A. J. Appi. Phys, 1996, 79, 2745; Parker, I. D. and Kim, H. H., “Fabrication of polymer light-emitting diodes using doped silicon electrodes,” Appl. Phys. Lett., 1994, 64, 1774). In addition to good coverage, thicker cathode layers were believed to provide self-encapsulation to keep oxygen and water vapor away from the active parts of the device.
It is known in the art that cathodes in the form of an ultrathin layer of either a low work function metal [see Cao, Y.; U.S. patent application Ser. No. 08/872,657 and Pichler, K., International patent application WO 98/10621] or an ultrathin layer of a low work function metal oxide [Cao, Y.; U.S. patent application Ser. No. 09/173,157 ]yield LEDs which offer comparable or better initial performance (e.g., brightness and efficiency) and extended operating lifetime, as compared to similar LEDs which employ conventional thick film cathodes.
Despite the improvements in the fabrication of polymer LEDs, issues remain to be resolved. For example, the brightness and efficiency of polymer LEDs are sufficient for them to be used in certain display applications. However, in battery operated devices, the luminous efficiency is a critical parameter. Higher luminous efficiency results directly in a longer period of use without recharging the battery. More generally, higher luminous efficiency enables use in a wider range of display applications. Thus, a need exists for polymer LEDs with higher luminous efficiency. In specific applications, the light output is preferred to be in a narrow cone in the forward direction. In such applications, high radiance is especially important.
For display applications, it is useful to measure light emitted as seen by a human observer, that is, in units that take into account the response of the human eye. Such units are called photopic units. One of the most commonly used photopic units to measure light output is candelas/Amp (cd/A). The light output in cd/A is a measure of the luminous efficiency.
The light output can also be measured in radiometric units, where the basic unit is the Watt per unit solid angle (W/sr/m2). Watts are absolute units because they are independent of wavelength; a Watt of visible light has the same power as a Watt of infrared light. In the Description of the Invention section, results from devices are quoted in both photometric and radiometric units, i.e., in Cd/A (luminous efficiency) and W/sr/m2 (radiance). The radiance is a measure of power delivered into the “forward direction”.
Jacobsen, S. M.; Jaffe, S. M.; Eilers, H.; Jones, M. J. U.S. Pat. No. 5,616,986 and Jacobsen, S. M.; Jaffe, S. M.; Eilers, H.; Jones, M. J. U.S. Pat. No. 5,469,018 teach the use of microcavity resonators with luminescent inorganic phosphor layers.
There have been many reports in the literature on microcavity devices fabricated with organic and polymer materials. Both metal etalon structures (where each electrode is a high reflectivity metal) and Distributed Bragg Reflector (DBR) structures (where one mirror is a stack of alternating high and low refractive index layers, called a DBR) have been employed. Microcavity structures are employed in conjugated polymer systems in order to tune the emission of the semi-conducting polymer to enable multi-color display applications (for example, see A. Dodabalapur, L. J. Rothberg, T. M. Miller and E. W. Kwock, Appl. Phys. Lett. 1994, 64, 2486; T. A. Fisher; D. G. Lidzey; M. A. Pate; M. S. Weaver, D. M. Whittaker, M. S. Skolnick, D. D. C. Bradley Appl. Phys. Lett. 1995, 67, 1355;Microcavity structures are also under investigation as resonant structures for lasers which utilize conjugated polymers as the gain materials [see for example, Tessler, N., Denton, G. J., and Friend, R. H., Nature, 1996, 382, 695; Diaz-Garcia, M. A.; Hide, F.; Schwartz, B. J.; McGehee, M. D.; Andersson, M. R.; Heeger, A. J., Appl. Phys. Lett. 1997, 70, 3191]. The resonant frequency of an optical microcavity is based on Equation (1):mλres =2Σinjdi  Equation (1)where λres is the resonant frequency of the cavity, m=the number of modes in the cavity, ni is the refractive index of the polymer layer i and di is the thickness of the polymer layer i. The index, ni of conjugated polymers depends on the band gap and molecular structure; typical values range from 1.8 to 2. As an example, if peak emission at 640 nm (λres=640 nm) is desired with ni=2.2, the total thickness of the polymer layers should be approximately 140 nm (at m=1). Thus, it is possible to tune the wavelength of the emitted light by changing the thickness of the layers between the two mirrors. It is on this property of optical microcavities that the multi-color display applications are based.
It Is generally accepted that microcavity structures fabricated with distributed Bragg reflector (DBR) mirrors are more desirable than metal etalon microcavities (see Wittman, H. F. et at, Adv. Mater. 1995, 7, 541) because there are fewer photons lost by absorption in the cavity. The losses in metal etalon structures are attributed to absorption by the metal layers. Tsutsui et al. reported that the emission intensity from a silver microcavity device was weaker than that from a non-microcavity device with ITO as the anode layer, even though the FWHM of emission was narrower in the silver device (N. Takada, T. Tsutsui, S. Saito, Appl. Phys. Lett. 1993, 63, 2032). In a subsequent paper dealing with DBR-based structures (T. Tsutsui, N. Takada, S. Saito, E. Ogino, Appl. Phys. Lett. 1994, 65, 1868) the authors refer to the poor operating lifetime obtained with silver devices relative to devices with ITO anodes. The poor performance is attributed to poor Ag film uniformity, and the authors state that they overcome the problem by using a DBR reflector with an ITO anode.
There are many reports of the use of high reflectivity, semi-transparent metal layers as one electrode in metal etalon microcavity structures. One measure by which microcavities are judged is Q, the quality of the cavity, as expressed in Equation (2):                     Q        =                  π                                    α              ⁢              1                        -                          ln              ⁢                                                                    R                    1                                    ⁢                                                                           ⁢                                      R                    2                                                                                                          Equation        ⁢                                   ⁢                  (          2          )                    where α1 refers to the internal absorption loss in the cavity and R1 and R2 are the reflectivities of the mirror surfaces on each side of the cavity
It is known that the quality, Q of the cavity is increased by increasing the thickness (reflectivity) of the mirrors. The higher Q is manifested by the narrowing of the spectral bandwidth (described as full-width at half-maximum, FWHM) of the light emitted. However, as one would expect there is a concomitant decrease in light output with increasing metal layer thickness. It is known that silver is a useful metal for the semi-transparent layer through which light is emitted. Silver is a high reflectivity metal that can be vapor deposited to give a smooth, continuous film. For example, see “Light-Emitting Conjugated Polymers in Optical Microcavities, Proceedings of the International Conference on Synthetic Metals, 1997, 84, 533 Cacialli, F.; Hayes, G. R.; Gruner, J.; Phillips, R. T.; Friend, R. H. In this case, the other side of the mirror is a DBR stack with an ITO electrode. No electroluminesence is measured in the device, but photoluminesence is reported.
The same configuration is also used in the polymer laser field. [See for example, Tessler, N., Denton, G. J., and Friend, R. H., Nature, 1996, 382, 695; Diaz-Garcia, M. A.; Hide, F.; Schwartz, B. J.; McGehee, M. D.; Andersson, M. R.; Heeger, A. J. Appl. Phys. Lett. 1997, 70, 3191].
According to Equation (2) above, the reflectivities of the mirrors should be as high as possible, in order to maximize Q. High reflectivity is essential to high Q microcavities. One would not expect to obtain high Q microcavities with mirrors fabricated from the low work-function metals or metal oxides that have been demonstrated as essential for obtaining efficient, bright and stable polymer LEDs. For example, barium metal has a reflectance of 39.8% at 578 nm (American Institute of Physics Handbook, 2nd Ed., McGraw-Hill Book Company, Chapter 6, “Optical Properties of Metals”, page 108).